Partial regeneration in a multi-level power inverter

ABSTRACT

In one embodiment, the present invention includes a medium voltage drive system having multiple power cells each to couple between a transformer and a load. A first subset of the power cells are configured to provide power to the load and to perform partial regeneration from the load, and a second subset of the power cells are configured to provide power to the load but not perform partial regeneration. A controller may be included in the system to simultaneously control a DC bus voltage of at least one of the first subset of the power cells, correct a power factor of the system, and provide harmonic current compensation for the system.

BACKGROUND

Generally, equipment referred to as a power converter, inverter or driveis used to provide power to another piece of equipment such as a motor.Specifically, such a converter (converter is used generally herein torefer to converters, inverters and drives) is coupled to a utilityconnection to receive incoming input power such as three-phase AC power.The converter conditions the power to provide a conditioned power signalto the equipment to be powered. In this way, incoming power to theequipment may be of improved efficiency, leading to reduced costs tooperate the equipment.

Multi-level power converters have been gaining popularity mainly due toimproved power quality, lower switching losses, better electromagneticcompatibility, and higher voltage capability. These improvements inpower conversion are achieved by using a multiple voltage step strategy.One common multi-level inverter topology is a series H-bridge inverter,in which multiple H-bridge inverters are connected in series. Since thistopology consists of series power conversion cells, the voltage andpower level may be easily scaled.

Typically, commercial converters are built up based on modular units,namely, power conversion cells, which are generally of a three-phasediode-based front-end rectifier, a DC-link capacitor bank, and asingle-phase full-wave inverter. Using such cells, improved powerquality at both the AC system and the motor sides can be realized.

However, this topology requires a large number of isolated DC voltagesources to supply each cell. The common practice is to use an isolationtransformer to supply a rectifier of a power cell. However, the supplycurrent to the rectifier contains many harmonic current components,which can be very disturbing for equipment and power systems, and causeelectromagnetic interference (EMI).

Further, the normal operation of the inverter in each cell generates alarge secondary current harmonic that is injected back into the DC-linkcapacitor. Thus a very large capacitor bank has to be used in order toreduce the voltage ripple. For a medium voltage drive operating at avoltage range of between approximately 4160 and 13800 volts, thiscapacitance bank can be on the order of between approximately 0.04 and0.5 Farads. Besides, the diode-based rectifier does not provide controlover the reactive input current component, and the diode-based rectifierdoes not provide the regenerative operating mode as required, forinstance, by downhill belt conveyors in mining applications, where thisoperating mode is the normal one, as several megawatts are required tobe taken back to the AC drive.

Voltage and current harmonics in power transmission and distributionhave become a serious problem. To limit the harmonic components of theinput current of the drive, often phase-shifted multi-windings isolationtransformers are used to supply power to the cells. However, to meet therequirements of the IEEE 519 standard, the impedance of the transformershould be high (typically on the order of approximately 8 to 15% PU formedium voltage drives) and a large amount of capacitance must beaccommodated in the DC bus of power cells, which make both transformersand power cells bulky and expensive.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a medium voltagedrive system that includes multiple power cells each to couple between atransformer and a load. A first subset of the power cells are configuredto provide power to the load and to perform partial regeneration fromthe load, and a second subset of the power cells are configured toprovide power to the load but not perform the partial regeneration. Acontroller may be included in the system to simultaneously control a DCbus voltage of at least one of the first subset of the power cells,correct a power factor of the system, and provide harmonic currentcompensation for the system.

Yet another aspect is directed to a method for receiving variousinformation in a partial regenerative drive system and controlling thesystem based on the information. Such information may include inputcurrent and voltage to a transformer, input current to an active frontend of a regenerative cell, and a bus voltage of the regenerative cell.From this information, a harmonic current reference, an active powercurrent reference, and a reactive power current reference can beindependently generated. Then a first portion of the harmonic currentreference can be combined with the active power current reference and asecond portion combined with the reactive power current reference. Fromthe combined current references, control signals can be generated forthe active front end.

Another aspect of the present invention is directed to a systemincluding transformers, phase output lines, and at least one controller.The transformers may include multiple modular transformers, where atleast one first modular transformer is not phase shifted and is coupledto a regeneration power cell, and at least one second modulartransformer that is phase shifted is coupled to a non-regeneration powercell. The cells may be coupled to phase output lines having at least oneregeneration power cell and non-regeneration power cell. In oneembodiment, the controller may simultaneously control a bus voltage of aregeneration power cell, correct a power factor of the system, andprovide harmonic current compensation for the system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an inverter in accordance with anembodiment of the present invention.

FIG. 2 is a functional block diagram of a control strategy in accordancewith one embodiment of the present invention.

FIG. 3 is a block diagram of a more detailed view of a reference signalgenerator in accordance with an embodiment of the present invention.

FIG. 4 is a block diagram of a drive in accordance with anotherembodiment of the present invention.

FIG. 5 is a flow diagram of a method in accordance with one embodimentof the present invention.

FIG. 6 is a block diagram of a system in accordance with anotherembodiment of the present invention.

DETAILED DESCRIPTION

Embodiments may provide a harmonics current-free partial regenerativehigh power medium voltage drive. By providing a drive having at leastsome regenerative power capability, higher performance can be realizedin applications that have faster deceleration times. Someimplementations can be used for applications such as test benches formotors and transmissions, oil pumps, heavy-duty cranes handling freightcontainers, centrifuges in food production and process industries,rolling mills, sheet-metal presses, cable-car controls, ski lifts, pumpcontrol at water treatment plants, and HVAC chiller control for officebuildings. In addition, regenerative power cells of the drive may alsoperform as an active filter and power factor corrector to compensate forharmonics currents as well as reactive power of the drive, which aregenerated by non-regenerative (e.g., diode front-end) power cells.

To enable such regeneration, embodiments may replace a passive (e.g.,diode-based) rectifier of one or more power cells with an activefront-end rectifier, allowing control of the active and reactive inputcurrent components and harmonics compensation. In addition, a controlstrategy to simultaneously control the DC-bus voltage, correct the powerfactor of the drive system, and compensate for harmonics current of thedrive system may be implemented. Since the active front-end converterscan compensate for harmonics of the passive rectifier currents, the sizeof capacitor banks in the individual cells and transformer impedance canbe reduced, resulting in lower costs and lower size of the individualcells. While the scope of the present invention is not limited in thisregard, in some medium voltage drive embodiments capacitance banks maybe between approximately 0.002 and 0.05 Farads, and the transformerimpedance less than approximately 7% PU. The control strategy alsoallows power recovery during, e.g., deceleration, by putting energy backinto the utility.

Referring now to FIG. 1, shown is a block diagram of an inverter inaccordance with an embodiment of the present invention. Morespecifically, FIG. 1 shows an implementation of a symmetrical cascadedmedium voltage inverter with partial regeneration capability for athree-phase motor. As shown in FIG. 1, inverter 100 may include modulartransformers 110 _(b) and 110 _(c) with passive phase shifting, realizedby both phase shifting of primary windings and secondary windings. Stillfurther, at least one other modular transformer 110 _(a) may be providedthat is not phase shifted. However, the outputs of this modulartransformer may be provided to power cells of a different configurationthan the other power cells. Specifically, these power cells 120_(a1)-120 _(c1) may be regenerative power cells having an active frontend, e.g., implemented by way of IGBTs 105. When these front end IGBTsare controlled accordingly, a relatively pure sinusoidal current in themain side (i.e., input current to the drive) having a minimal harmonicdistortion is realized.

Nonetheless, the winding sets of the primary and secondary transformermodules 110 _(b) and 110 _(c) that supply power to the other power cellscan be phase shifted to minimize the harmonics in the main current.Implementations of such phase-shifting transformers are described inmore detail in U.S. patent application Ser. Nos. 12/284,649 and12/284,654, commonly assigned herewith, the disclosures of which arehereby incorporated by reference. In this example, the primaries arephase shifted by 10° and the secondaries are phase shifted by 20°. Thusin the embodiment of FIG. 1, an equivalent 36-pulse transformer can berealized using two 18-pulse transformers 110 _(b) and 110 _(c) withphase shifted primaries. However, the phase shifting is not needed forthe transformer module 110 _(a), as the electronically controlled activefront-end 105 of the power cell will compensate for harmonics of itselfand remaining harmonics of the non-regenerative cells. Hence the MVdrive draws nearly pure sinusoidal current from the utility.

Furthermore, by providing an active front end, this implementationprovides the ability for partial regeneration. Of course, otherimplementations are possible using different combinations of active andpassive transformers, as well as control means for actively controllingone or more power cells. Note that in the embodiment of FIG. 1, acontroller 180 may be coupled to the power cells (note the connectionsare not shown in FIG. 1 for ease of illustration). This controller isrepresentative, and in particular implementations multiple suchcontrollers may be provided, e.g., local controllers associated with oneor more power cells, and a master controller to control the drive systemas a whole. Furthermore, this controller may provide control of theactive switching of the front end IGBTs of power cells 120 _(a1)-120_(c1) to enable a nearly pure sine wave input current to the drive, aswell as to enable a partial regeneration mode.

In the configuration of FIG. 1, three symmetrical cells in series (e.g.,cells 120_(a1)-120 _(a3)) may form one motor phase voltage. The actualnumber of series-connected cells is determined by the required loadvoltage and power. The phase voltages of the load (e.g., motor 130) arethe summation of the single-phase voltage generated by each cell. In theexample shown in FIG. 1, active front-end converters 120 _(a1)-120 _(c1)are used to supply power to one cell of each phase. Other power cells120 _(a2)-120 _(a3) have passive diode front-end rectifiers. As shown inthis embodiment, each power cell 120 can be an H-bridge inverter,although the scope of the present invention is not limited in thisregard. This configuration will allow up to 33% regeneration power,i.e., one of the three power cells in each phase line is capable ofregeneration. However, if the regenerative cell size is larger than thenon-regenerative cell size, the amount of regeneration power can behigher.

As mentioned above, a control strategy in accordance with an embodimentof the present invention can simultaneously compensate for currentharmonics of the drive, regulate the DC-bus voltage of the suppliedcell, correct the power factor of the drive system, and control therecovered power. In so doing, the need for a front end filter coupledbetween utility connection and input to the transformer can be avoided.That is, rather than a conventional system that implements an activefilter (which is typically a separate component coupled between utilityand transformer), drive systems as disclosed herein can be directlycoupled to a utility connection. As such the expense and complexity ofan added front end component to perform harmonic compensation can beavoided.

Referring now to FIG. 2, shown is a functional block diagram of acontrol strategy in accordance with one embodiment of the presentinvention. For simplicity, only one regeneration power cell 120 _(a1)has been shown, along with a single passive power cell 120 _(m1). Asshown in FIG. 2, system 200 includes a controller 210. In variousembodiments, controller 210 may be a digital controller that can beimplemented using hardware, software, firmware or combinations thereof.As examples, controller 210 may be a digital signal processor (DSP), amicrocontroller, a field programmable gate array (FPGA), a dedicated orgeneral-purpose microprocessor or the like. Note that controller 210 maybe a local controller only to control the active front end 105 of powercell 120. Multiple such controllers may be provided, one for eachindividual active regeneration power cell. While not shown, in someimplementations a separate controller may be present to control the Hbridge 108 of the corresponding regeneration power cell. Similar suchcontrollers may be provided for each of the passive power cells, e.g.,power cell 120 _(m1). In yet additional embodiments, another controllercan be provided, such as a master controller that in turn can providecontrol signals to controller 210, e.g., to disable certain correctionand compensation features during transient load conditions.

As shown in FIG. 2, controller 210 may include various components, whichmay be formed of dedicated hardware and/or programmable control modules.In the implementation shown in FIG. 2, these components may include ananalog-to-digital converter (ADC) 212, which is coupled to receive anddigitize various inputs. As seen in FIG. 2, in one embodiment, theDC-link voltage, input currents to regeneration cell 120 _(a1), inputcurrents and voltages to the drive (e.g., from the utility or otherinput power source) are sensed through voltage and current sensors (notshown for ease of illustration in FIG. 2) and can be converted todigital format in ADC 212. Other sensing mechanisms and monitoringinformation can be used in other embodiments.

The digitized data may then be provided to a reference signal generator214, which may implement one or more control algorithms in accordancewith an embodiment of the present invention. In turn, the output ofreference signal generator 214, which may be a plurality of voltagereference signals, e.g., a single voltage reference signal for eachphase of the regeneration power cell to be controlled, may be providedto a pulse width modulation (PWM) modulator 216. From these voltagereference signals, PWM modulator 216 may generate control signals (e.g.,gate signals) to control the active front-end of regeneration cell 120,namely switching signals to control IGBTs 105. While shown with thisparticular implementation in FIG. 2, understand that in someimplementations at least portions of the controller 200 can be realizedas computer-readable instructions that are stored on a tangible storagemedium, such as instructions stored in a memory for reading by aprocessor such as a DSP or programmable processor to perform thedescribed operations.

Referring now to FIG. 3, shown is a block diagram of a more detailedview of a reference signal generator in accordance with an embodiment ofthe present invention. As an example, reference signal generator 300 maytake the form of reference signal generator 214 in FIG. 2. In general,reference signal generator 300 may include a current reference generator310 which, based on the inputs, may provide reference currents to avoltage reference generator 360 that in turn provides the voltagereference signals to a control mechanism, e.g., PWM modulator 216 ofFIG. 2.

Current reference generator 310 may include independent control andcompensation mechanisms for all of DC voltage control, harmonic currentcompensation, and power factor correction. More specifically, currentreference generator 310 may include a DC voltage controller 320, aharmonics compensator 330, and a power factor corrector 340. Theresulting outputs of these independent control and compensationmechanisms may be appropriately combined by way of summing blocks and amatrix transformer 350. As will be described below, some implementationsmay selectively enable/disable at least one of the harmonic currentcompensation and the power factor correction, possibly under transientload conditions.

As seen, the variable information obtained via the voltage and currentsensors may be used in current reference generator 310. Specifically,the DC bus voltage (V_(DC(AI))) obtained from the DC bus voltage ofregeneration cell 120 _(A1) may be provided to DC voltage controller320. In turn, the input current obtained, e.g., from the utility, isprovided to harmonics compensation 330. Power factor corrector 340further receives the input voltage to the transformer. To achieve thedecoupled control of active, reactive and harmonics currents, thevariables can be transformed to two phase q-d rotating reference framerotating at angular displacement of supply Θ. This arbitrary q-dreference frame is explained using the following equations:

$\begin{matrix}{{\begin{bmatrix}f_{q} \\f_{d} \\f_{0}\end{bmatrix} = {{T(\Theta)} \cdot \begin{bmatrix}f_{a} \\f_{b} \\f_{c}\end{bmatrix}}}{where}} & \left\lbrack {{Eq}.\mspace{14mu} 1} \right\rbrack \\{{T(\Theta)} = \begin{bmatrix}{\cos \; \Theta} & {\cos \left( {\Theta - \frac{2\; \pi}{3}} \right)} & {\cos \left( {\Theta + \frac{2\; \pi}{3}} \right)} \\{\sin \; \Theta} & {\sin \left( {\Theta - \frac{2\; \pi}{3}} \right)} & {\sin \left( {\Theta + \frac{2\; \pi}{3}} \right)} \\\frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}} & \frac{1}{\sqrt{2}}\end{bmatrix}} & \left\lbrack {{Eq}.\mspace{14mu} 2} \right\rbrack\end{matrix}$

Note that T(Θ) is the transformation matrix and f represents currents orvoltages. This transformation transfers the three-phase stationaryparameters, f_(a), f_(b), f_(c) from a-b-c system to two-phaseorthogonal rotating reference frame. However, this transformation may beperformed in two steps to transfer the parameters to rotating excitationreference frame q^(e)-d^(e). The first step is to transfer three phasestationary parameters f_(a), f_(b), f_(c) from a-b-c system to two-phaseorthogonal stationary reference frame q^(s)-d^(s) by substituting θ=0 inEq.2 (and shown in transformation matrices 334 and 346). In the secondtransformation the vectors in 2-phase orthogonal stationary system f_(q)^(s) and f_(d) ^(s) are converted to orthogonal reference frame f_(q)^(e) and f_(d) ^(e) (and shown in transformation matrices 335 and 348).This transformation can be shown by the following equations:

$\begin{matrix}{f_{q}^{e} = {{\cos \; {\theta_{e} \cdot f_{q}^{s}}} - {\sin \; {\theta_{e} \cdot f_{d}^{s}}}}} & \left\lbrack {{Eq}.\mspace{14mu} 3} \right\rbrack \\{f_{d}^{e} = {{\sin \; {\theta_{e} \cdot f_{q}^{s}}} + {\cos \; {\theta_{e} \cdot f_{d}^{s}}}}} & \left\lbrack {{Eq}.\mspace{14mu} 4} \right\rbrack \\{\theta_{e} = {\tan^{- 1}\frac{f_{d}^{s}}{f_{q}^{s}}}} & \left\lbrack {{Eq}.\mspace{14mu} 5} \right\rbrack\end{matrix}$

In general in this reference frame, active power or DC-bus voltage iscontrolled by the q-axis current component, and reactive power or powerfactor is controlled by the d-axis current component. The active powerand reactive power in two-phase excitation reference frame can becalculated as follows:

$\begin{matrix}{P = {\frac{3}{2}{v_{q}^{e} \cdot i_{q}^{e}}}} & \left\lbrack {{Eq}.\mspace{14mu} 6} \right\rbrack \\{Q = {{- \frac{3}{2}}{v_{q}^{e} \cdot i_{d}^{e}}}} & \left\lbrack {{Eq}.\mspace{14mu} 7} \right\rbrack\end{matrix}$

where v_(q) ^(e) is the quadrature component of voltage in two-phaseexcitation reference frame, i_(q) ^(e) the quadrature component ofcurrent in two-phase excitation reference frame and i_(d) ^(e) thedirect component of current in two-phase excitation reference frame. Forthree-phase balanced system, v_(q) ^(e) is equal to peak magnitude ofvoltages in a-b-c system. Hence, the active power can be controlled byi_(q) ^(e) and reactive power can be controlled by i_(d) ^(e). Thecurrent references in excitation two-phase reference frame (i_(q) ^(e)*and i_(d) ^(e)*) can be generated using the following equations:

_(q) ^(e) * =Δi* _(q) +i _(qh) ^(e)*   [Eq. 8]

i _(d) ^(e) *=Δi* _(d) +i _(dh) ^(e)*   [Eq. 9]

The DC bus voltage or active power is controlled by the Δi*_(q)component, and power factor or reactive power is controlled by theΔi*_(d) component. Further, harmonics compensation can be controlled byi_(qh) ^(e)* and i_(dh) ^(e)* components

As seen in FIG. 3, Δi*_(q) is generated in DC voltage controller 320 bysubtraction of a DC-bus voltage command V_(dc)* signal, which may be setby user, from the DC-bus voltage feedback (V_(dc(a1))) in a summer 322.The result is then passed through a proportional-integral (PI)controller 324. In turn, Δi*_(d) in power factor corrector 340 can becalculated using Eq.7 in following manner:

$\begin{matrix}{{\Delta \; i_{d}^{*}} = {\frac{2}{3N}{v_{q}^{e} \cdot Q^{*}}}} & \left\lbrack {{Eq}.\mspace{14mu} 10} \right\rbrack\end{matrix}$

where N is turns ratio of transformer 110 a. Q* is reactive powercommand, and v_(q) ^(e) is quadrature component of input voltage todrive in two-phase excitation reference frame. Q* can be set by user asarbitrary value or can be instantaneous reactive power into the drive.

To perform harmonics current compensation control, harmonics currentreferences may be generated in two-phase rotating reference frame. Thecurrents of phase R (I_(R)) and phase S (I_(S)) of the drive can besensed through current sensors or via a current transformer (CT). Assumethat the turns ratio (which can be set in a turns ratio calculator 332)of the isolation transformer is N and number of regenerative units isequal to m, the input current to regeneration cell 120 can be estimatedas:

$\begin{matrix}{i_{R{({{Sec}.})}} = {\frac{N.}{m}I_{R}}} & \left\lbrack {{Eq}.\mspace{14mu} 11} \right\rbrack \\{i_{s{({{Sec}.})}} = {\frac{N.}{m}I_{s}}} & \left\lbrack {{Eq}.\mspace{14mu} 12} \right\rbrack\end{matrix}$

where i_(R(Sec.)) and i_(s(Sec.)) are transformed to two-phaseexcitation reference frame in a matrix transformers 334 and 335 usingEquations 1 through 5 to generate i_(q) ^(e) and i_(d) ^(e). Theharmonics current reference frame in two-phase rotating reference framei_(qh) ^(e)* and i_(dh) ^(e)* can then be generated by passing i_(q)^(e) and i_(d) ^(e) through a high pass filter 336 to reject the DCcomponents.

Thus the outputs from harmonics compensator 330, namely i_(qh) ^(e)* andi_(dh) ^(e)* may be summed with the active power current reference,ΔI*_(q), obtained from voltage controller 320 and the reactive powercurrent reference, ΔI*_(d), obtained from power factor corrector 340,via summers 345 a and 345 b, respectively. These combined referencecurrents then may be provided to another transformation matrix 350,which takes the d-q reference frame components and converts them back tothree-phase components. As seen, these reference current outputs may beoutput to voltage reference generator 360.

These three phase reference currents, i*_(a), i*_(b), i*_(c), and sensedcurrent values from the active front end may be the inputs to voltagereference generator 360, which may generate voltage reference signals.As seen, in FIG. 3, the three-phase current references (i_(a)*, i_(b)*,i_(c)*) are provided to voltage reference generator 360, and from whichare subtracted feedback currents of the front-end converter ofregeneration cell 120 _(a1) (i_(A(invA1)), i_(B(invA1)), i_(C(invA1)))in corresponding summers 362 a-362 c. The results are passed throughcorresponding PI controllers 365 a-365 c to generate the three phasevoltage references (V_(A*(ReGenA1)), V_(B*(ReGenA1)), V_(C*(ReGenA1))),where V_(A*(ReGenA1)) is the voltage reference for phase A ofregeneration cell 120 _(A1); V_(B*(ReGenA1)) is the voltage referencefor phase B of regeneration cell A1, and V_(C*(ReGenA1)) is voltagereference for phase C of regeneration cell 120 _(A1). In this way,control of active power, reactive power and harmonics compensation ofthe drive are decoupled and can be simultaneously done.

FIG. 4 is a block diagram of a drive in accordance with anotherembodiment of the present invention. In this example, a single non-phaseshifting isolating transformer 110 is used. The same control strategydescribed above can be applied to control the active front-ends 105 ofregeneration cells 120 _(a1-c1). Hence, the power factor and harmonicscurrent of the other passive front rectifiers of power cells 120_(a2-c3) can be compensated by these active front-end converters. Inthis way, the input current to drive system 100′ can be made harmonicsfree. Note that embodiments can be applied to both symmetric orasymmetric cascaded multi-level inverters.

Referring now to FIG. 5, shown is a flow diagram of a method inaccordance with one embodiment of the present invention. Method 500 maybe implemented using individual local controllers for controlling thefront end of each regeneration cell, as well as potentially to controlthe back end H-bridge of such regeneration cells. In addition, localcontrollers can handle back end switching of non-regeneration powercells. Still further, a master controller may handle some or all of thesteps set forth, in certain implementations.

As shown in FIG. 5, method 500 may begin by receiving various inputinformation from a drive system (block 510). More specifically, thisinformation may include input current and voltage from a transformerinput line, input current from an active front end of a regenerationcell, as well as the cell's DC bus voltage. Based on this information, aharmonic current reference, active power current reference, and reactivepower current reference may be simultaneously and independentlygenerated (block 520). However, understand that as discussed above,e.g., based on transient mode conditions, one or more of the harmoniccurrent reference and reactive power current reference may be at leasttemporarily disabled. Based on the information obtained, portions of theharmonic current reference may be combined with the active power currentreference and reactive power current reference, respectively (block530). Based on this information, combined current references may becomputed in combination with input current feedback information,obtained from the active front end (block 540). Finally, voltagereference signals may be generated from the combined current referencesin block 550. More specifically, such voltage reference signals may beused to generate gate signals to control the active front end of a givenregeneration power cell. While shown with this particular implementationin the embodiment of FIG. 5, understand the scope of the presentinvention is not limited in this regard.

Referring now to FIG. 6, shown is a block diagram of a system inaccordance with another embodiment of the present invention. As shown inFIG. 6, system 600 may be a medium-voltage drive including both activeand passive power cells. Specifically, in the embodiment of FIG. 6, athree-phase, medium-voltage drive is shown that includes a plurality ofactive front-end power cells 620 _(A1)-620 _(C1) and multiple passivefront-end power cells, namely power cells 620 _(A2)-620 _(C3).

As seen, different types of local controllers may be present to controlthese different power cells. Specifically, the active power cellsinclude an active front controller 625 _(A1)-625 _(A3) and a local cellcontroller 626 _(A1)-626 _(A3), which are to control the front end andback end switching elements of the power cells, respectively. Instead,passive power cells 620 _(A2)-620 _(C3) include only a single local cellcontroller 626 _(A2)-626 _(C3).

As seen, each of these local controllers may communicate with a fiberoptic interface 660. In some implementations, a pair of unidirectionalfiber optic channels may be coupled between each local controller andfiber optic interface 660. In turn, fiber optic interface 660communicates with a master controller 640 that further includes an ADC645.

Master controller 640 may provide control signals to fiber opticinterface 660 for transmission to the different local controllers. Inone embodiment, these control signals may be voltage reference signals,which cause the local controllers to perform certain processing togenerate the needed switching signals. In other implementations, theswitching signals themselves may be sent by master controller 640 fortransmission to the local cell controllers.

As further seen in FIG. 6, a signal conditioning board 650 may bepresent to sense or perform signal processing with regard to variousinformation, namely voltage and/or current information obtained bothfrom the input power source and the output of the different phase outputlines coupled to a load 630 which in one embodiment may be a motor.

In addition to the control information described above, additionalinformation from master controller 640 can be provided to the individuallocal controllers. In addition, the local controllers can provideinformation such as status information, both as to normal operation aswell as faults, over-temperature situations or so forth, back to mastercontroller 640. Master controller 640 may further be associated with auser input device 655 such as a keyboard and/or touch screen display toenable user input to control various features such as speed, torque,selection of different power cells to be enabled and so forth, as wellas to provide status information to the user via a given display orother output means. While shown with this particular implementation inthe embodiment of FIG. 6, the scope of the present invention is notlimited in this regard.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A medium voltage drive system comprising: a plurality of power cellseach to couple between a transformer and a load, wherein a first subsetof the power cells are configured to provide power to the load and toperform partial regeneration from the load, and a second subset of thepower cells are configured to provide power to the load but not performthe partial regeneration.
 2. The system of claim 1, further comprising acontroller to simultaneously control a DC bus voltage of at least one ofthe first subset of the power cells, correct a power factor of thesystem, and provide harmonic current compensation for the system.
 3. Thesystem of claim 2, wherein each of the first subset of the power cellsincludes an active front end to receive gate signals from thecontroller.
 4. The system of claim 3, wherein the controller includes adigitizer to digitize voltage signals and current signals from an inputto the transformer, input current signals to the active front end, andthe DC bus voltage and provide the digitized information to a referencesignal generator, the reference signal generator to generate referencesignals to provide to a modulator, the modulator to generate the gatesignals based on the reference signals.
 5. The system of claim 4,wherein the controller includes a harmonic compensator to generate aharmonic current reference, a voltage controller to generate an activepower current reference, and a power factor corrector to generate areactive power current reference.
 6. The system of claim 5, wherein thecontroller is to generate a first current reference signal fromcombination of a first portion of the harmonic current reference and theactive power current reference, and a second current reference signalfrom combination of a second portion of the harmonic current referenceand the reactive power current reference.
 7. The system of claim 2,further comprising a second controller to control switching of a backend of at least one of the first subset of the power cells.
 8. Thesystem of claim 2, further comprising a master controller to generatecontrol signals to selectively disable at least one of the power factorcorrection and the harmonic current compensation controlled by thecontroller.
 9. The system of claim 8, wherein the master controller isto selectively disable based on transient conditions of the load.
 10. Amethod comprising: receiving information regarding an input current andinput voltage to a transformer of a partial regenerative drive system,regarding an input current to an active front end of a regenerative cellof the partial regenerative drive system, and regarding a bus voltage ofthe regenerative cell; independently generating a harmonic currentreference, an active power current reference, and a reactive powercurrent reference from the information; combining a first portion of theharmonic current reference with the active power current reference andcombining a second portion of the harmonic current reference with thereactive power current reference; and generating control signals for theactive front end using the combined current references.
 11. The methodof claim 10, further comprising determining a secondary current from theinput current to the transformer and converting the secondary current toa two-phase rotating reference frame, and filtering the secondarycurrent to generate the harmonic current reference.
 12. The method ofclaim 11, further comprising generating the reactive power currentreference based on the input current and input voltage to thetransformer.
 13. The method of claim 12, further comprising combiningthe bus voltage information and a reference bus voltage and processingthe combined voltage in a proportional-integral controller to generatethe active power current reference.
 14. The method of claim 13, furthercomprising combining the combined current references with theinformation regarding the input current to the active front end.
 15. Themethod of claim 10, further comprising selectively disabling at leastone of the reactive power current reference generation and the harmoniccurrent reference generation based at least in part on transientconditions of a load coupled to the partial regenerative drive system.16. A system comprising: a plurality of modular transformers, wherein atleast one first modular transformer includes a non-phase-shifted primarywinding coupled to an input power source and non-phase-shifted secondarywindings each coupled to a regeneration power cell, and at least onesecond modular transformer including a phase-shifted primary windingcoupled to the input power source and a plurality of phase-shiftedsecondary windings each coupled to a non-regeneration power cell; afirst phase output line having at least a regeneration power cell and anon-regeneration power cell; a second phase output line having at leasta regeneration power cell and a non-regeneration power cell; a thirdphase output line having at least a regeneration power cell and anon-regeneration power cell; and a controller to simultaneously controla bus voltage of a regeneration power cell, correct a power factor ofthe system, and provide harmonic current compensation for the system.17. The system of claim 16, wherein the controller includes: a digitizerto digitize voltage signals and current signals from the input powersource, input current signals to the regeneration power cell, and thebus voltage of the regeneration power cell; and a reference signalgenerator to receive the digitized information to generate referencesignals to provide to a modulator to generate gate signals for a frontend of the regeneration cell based on the reference signals.
 18. Thesystem of claim 16, wherein the controller includes a harmoniccompensator to generate a harmonic current reference, a voltagecontroller to generate an active power current reference, and a powerfactor corrector to generate a reactive power current reference.
 19. Thesystem of claim 16, wherein the controller comprises a local controllerassociated with the regeneration power cell of the first phase outputline to control a front end of the regeneration power cell, and furthercomprising a master controller coupled to the local controller, whereinthe master controller is to generate control signals to selectivelydisable at least one of the power factor correction and the harmoniccurrent compensation controlled by the local controller.
 20. The systemof claim 19, wherein the local controller comprises a first localcontroller to control the regeneration power cell front end and a secondlocal controller to control a back end of the regeneration power cell.